De Beers also says it has leant on wholesale purchasers who buy from its stockpile, but also buy diamonds on the open market, to follow suit.

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"America is the most important diamond market in the world, " says Andrew Coxon, the president of De Beers Institute of Diamonds and its top diamond buyer.

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He says he is willing to reconsider his firm's holding in De Beers if it does not start performing better, but he also hints at increasing his stake.

ECONOMIST: The inside outsider | The